The means by which a warm blooded animal overcomes microbial pathogenesis is a complex process. Immunity to microbial pathogenesis is one means by which a warm blooded animal avoids pathogenesis, or suffers a less intense pathogenic state. Incomplete immunity to a given pathogen results in morbidity and mortality in a population exposed to a pathogen.
Achieving an immune state equal to the accelerated secondary immune response following reinfection with a pathogenic microorganism has been a goal of public health officials. This immune response, often achieved only following clinically significant microbial pathogenesis, is sought to be induced by vaccines. Unfortunately, currently available vaccines fall short of this goal. Thus, the accelerated secondary immune response is often found only after the host organism has suffered the disease state.
Vaccines for the purpose of conferring immunity upon a host organism are, of course, known. Vaccines that confer immunity to microbial infections can contain live, attenuated or killed microorganisms, depending upon the type of vaccine. However, the degree of protection conferred by these types of vaccines is highly variable.
It is generally agreed that in the case of intracellular pathogens vaccines based on live but attenuated microorganisms (live vaccines) induce a highly effective type of immune response. Such vaccines have the great advantage that, once the animal host has been vaccinated, entry of the microbial pathogen into the host induces an accelerated recall of earlier, cell-mediated or humoral immunity which is able to control the further growth of the organism before the infection can assume clinically significant proportions. Vaccines based on a killed pathogen (killed vaccine) are generally conceded to be unable to achieve this type of response. However, vaccines that contain a live pathogen present the danger that the vaccinated host upon vaccination may contract the disease against which protection is being sought.
It would be desirable to have a vaccine that possesses the immunizing attributes of a live vaccine but that is not capable of causing an undesirable infection upon vaccination. To this end, a vaccine based on a non-virulent, auxotrophic strain of Salmonella typhimurium has been utilized as an experimental immunogen. [Hoiseth et al., Nature 291:238-239 (1981).]However, the described auxotrophic mutant was derived from a model bacteria strain of artificially-maintained virulence, not from a naturally-occurring pathogenic bacteria. In addition, an auxotrophic strain, requiring a metabolite ordinarily unavailable in tissue of the animal to be immunized, may not be able to survive in the animal to be immunized for a time period long enough to induce the desired immunity.
As pointed out hereinabove, microbial agents of disease often present some of the most serious clinical consequences, but only incomplete protection against such agents is provided by currently available vaccines. Furthermore, the lack of reliable in vitro testing in animal models makes it difficult to develop guidelines for the quality control of these vaccines when manufactured in commercial quantities. A method aspect of the present invention mitigates these problems in that a phagocytic cell assay is provided that allows for a relatively easy and reliable identification and selection of microbial strains that are ideal candidates for live vaccines and that confer an adequate level of immunity in the animal sought to be protected. Also, the selected strains are incapable in the first instance of infecting the host with disease-causing pathogens.
Macrophage assays have been used heretofore to determine the sensitivity of macrophages to a given virulent strain of bacteria. [Lissner et al., J. Immun. 131(6):3006-3013 (1983).]Phagocytosis of bacteria by particular types of macrophages has been studied to determine the bactericidal abilities of macrophages from particular subspecies of experimental animals. However, macrophage assays heretofore have not been used to screen for avirulent strains of microbial pathogens.